1. Field of the Invention
The invention relates to tissue core biopsy instruments. More particularly, the invention relates to a tissue core biopsy cannula having an improved geometry at its needle point.
2. State of the Art
The biopsy procedure involves the diagnostic evaluation of sample cells which are removed from a patient. There are many different types of instruments used to obtain sample cells for analysis. One common instrument which is used to obtain soft tissue samples is a coring cannula having a needle point. The coring cannula may be manually operated or may be attached to a mechanical device which permits semi-automatic operation. In either case, the geometry of the cannula and its needle point are functionally the same. In operation, the coring cannula is typically fitted with a stylet which substantially occludes the hollow interior of the coring cannula. The stylet is usually provided with a sharpened distal end which facilitates insertion of the cannula-stylet assembly into the body. The stylet prevents unwanted tissue from entering the cannula while the cannula is moved through the body to the target tissue.
According to one type of biopsy instrument, the stylet is provided with a sampling notch proximal of its sharpened point. Once the cannula-stylet assembly has reached the target tissue, the stylet is moved distally out of the cannula exposing the sampling notch to the target tissue. A portion of target tissue prolapses and fills the region defined by the notch in the stylet. The cannula is then moved distally over the notch in the stylet, severing the sample and capturing it. The cannula-stylet assembly is then removed from the body with the sample securely held inside the cannula. While effective, this type of biopsy instrument only permits a relatively small sample to be taken, limited in size by the length and depth of the notch.
Another type of biopsy instrument includes a solid stylet. When the cannula-stylet assembly has reached the target tissue, the stylet is withdrawn from the cannula. The cannula is then advanced into the target tissue so that its needle point cuts the target tissue and a relatively large core sample of the target tissue is captured in the hollow interior of the cannula. If successful, the core sample is still connected to adjacent target tissue distal of the needle point of the cannula, however. If the cannula is merely withdrawn from the target tissue, it is possible that the core sample will remain connected to the adjacent target tissue and will evade capture by the cannula.
Several methods are used to prevent the core sample from exiting the cannula. One method is to apply negative pressure at the proximal end of the cannula while withdrawing the cannula from the target tissue. Another method is to provide the cannula with a non-cylindrical lumen such that when the cannula is rotated, the captured tissue is rotated relative to the adjacent tissue to sever the sample from the adjacent tissue.
While it is important to take measures to keep the sample in the cannula while the cannula is withdrawn, it is equally important to assure that the sample is actually captured by the cannula. The cannulae used in tissue core biopsy instruments are typically relatively small, having a diameter in the range of one-half to six millimeters. Clearly, the smaller the cannula diameter, the less disruptive the biopsy procedure will be. However, when the cannula diameter is small, there is the possibility that tissue will spread or deflect away from the cannula point rather than enter the interior of the cannula and be captured.
The needle point of a typical biopsy cannula 10 is shown in prior art FIG. 1. The distal end 12 of the cylindrical cannula 10 is ground in a single plane P which lies at an angle .alpha. relative to the longitudinal axis 14 of the cannula 10. This produces an elliptical opening 16 at the distal end 12 of the cannula 10. The opening 16 is defined by the side wall 18 of the cannula 10 and provides an entrance to the lumen 20 of the cannula 10. The most distal portion of the side wall 18 forms the cutting edge 22. By choosing an appropriate angle .alpha., e.g. 20.degree., the cutting edge 22 can be made very sharp. As shown in FIG. 1a, when the distal end 12 of the cannula 10 is advanced into target tissue 28, the tissue 28 is severed by the cutting edge 22 and follows a paths relative to the opening 16 as shown by the arrow 24 in FIG. 1. Ideally, the tissue enters the opening 16, but depending on the properties of the tissue 28, the thickness of the sidewall 18, and the angle .alpha., the tissue may merely be deflected away from the opening 16. Since the sharpest part of the cannula is the relatively small cutting edge 22, it is possible that the severed tissue will be guided by the sidewall 18 along a path lying in the plane P and never enter the opening 16.